The present invention relates to the orthopedic field and the provision of prostheses, such as hip and knee implants, as well as methods of manufacture of such devices and material used therein.
The use of synthetic polymers, e.g., ultra high molecular weight polyethylene, with metallic alloys has revolutionized the field of prosthetic implants, e.g., their use in total joint replacements for the hip or knee. Wear of the synthetic polymer against the metal of the articulation, however, can result in severe adverse effects which predominantly manifest after several years. Various studies have concluded that such wear can lead to the liberation of ultrafine particles of polyethylene into the periprosthetic tissues. It has been suggested that the abrasion stretches the chain folded crystallites to form anisotropic fibrillar structures at the articulating surface. The stretched-out fibrils can then rupture, leading to production of submicron sized particles. In response to the progressive ingress of these polyethylene particles between the prosthesis and bone, macrophage-induced resorption of the periprosthetic bone is initiated. The macrophage, often being unable to digest these polyethylene particles, synthesize and release large numbers of cytokines and growth factors which can ultimately result in bone resorption by osteoclasts and monocytes. This osteolysis can contribute to mechanical loosening of the prosthesis components, thereby sometimes requiring revision surgery with its concomitant problems.
It is an object of the invention to provide an implantable prosthesis device formed at least in part of radiation treated ultra high molecular weight polyethylene (UHMWPE) having no detectable free radicals, so as to reduce production of fine particles from the prosthesis during wear of the prosthesis.
It is another object of the invention to reduce osteolysis and inflammatory reactions resulting from prosthesis implants.
It is yet another object of the invention to provide a prosthesis which can remain implanted within a person for prolonged periods of time.
It is yet another object of the invention to provide improved UHMWPE which can be used in the prostheses of the preceding objects and/or in other fabricated articles.
Still another object of the invention is to provide improved UHMWPE which has a high density of cross-links and no detectable free radicals.
A still further object of the invention is to provide improved UHMWPE which has improved wear resistance.
According to the invention, a medical prosthesis for use within the body which is formed of radiation treated ultra high molecular weight polyethylene (UHMWPE) having substantially no detectable free radicals, is provided. The radiation can be, e.g., gamma or electron radiation. The UHMWPE has a cross-linked structure. Preferably, the UHMWPE is substantially not oxidized and is substantially oxidation resistant. Variations include, e.g., the UHMWPE having three melting peaks, two melting peaks or one melting peak. In certain embodiments, the UHMWPE has a polymeric structure with less than about 50% crystallinity, less than about 290 xc3x85 lamellar thickness and less than about 940 MPa tensile elastic modulus, so as to reduce production of fine particles from the prosthesis during wear of the prosthesis. Part of the prosthesis can be, e.g., in the form of a cup or tray shaped article having a load bearing surface made of this UHMWPE. This load bearing surface can be in contact with a second part of the prosthesis having a mating load bearing surface of a metallic or ceramic material.
Another aspect of the invention is radiation treated UHMWPE having substantially no detectable free radicals. This UHMWPE has a cross-linked structure. Preferably, this UHMWPE is substantially not oxidized and is substantially oxidation resistant. Variations include, e.g., the UHMWPE having three melting peaks, two melting peaks or one melting peak.
Other aspects of the invention are fabricated articles, e.g., with a load bearing surface, and wear resistant coatings, made from such UHMWPE. One embodiment is where the fabricated article is in the form of a bar stock which is capable of being shaped into articles by conventional methods, e.g., machining.
Yet another aspect of the invention includes a method for making a cross-linked UHMWPE having substantially no detectable free radicals. Conventional UHMWPE having polymeric chains is provided. This UHMWPE is irradiated so as to cross-link said polymeric chains. The UHMWPE is heated above the melting temperature of the UHMWPE so that there are substantially no detectable free radicals in the UHMWPE. The UHMWPE is then cooled to room temperature. In certain embodiments, the cooled UHMWPE is machined and/or sterilized.
One preferred embodiment of this method is called CIR-SM, i.e., cold irradiation and subsequent melting. The UHMWPE that is provided is at room temperature or below room temperature.
Another preferred embodiment of this method is called WIR-SM, i.e., warm irradiation and subsequent melting. The UHMWPE that is provided is pre-heated to a temperature below the melting temperature of the UHMWPE.
Another preferred embodiment of this method is called WIR-AM, i.e., warm irradiation and adiabatic melting. In this embodiment, the UHMWPE that is provided is pre-heated to a temperature below the melting temperature of the UHMWPE, preferably between about 100xc2x0 C. to below the melting temperature of the UHMWPE. Preferably, the UHMWPE is in an insulating material so as to reduce heat loss from the UHMWPE during processing. The pre-heated UHMWPE is then irradiated to a high enough total dose and at a fast enough dose rate so as to generate enough heat in the polymer to melt substantially all the crystals in the material and thus ensure elimination of substantially all detectable free radicals generated by, e.g., the irradiating step. It is preferred that the irradiating step use electron irradiation so as to generate such adiabatic heating.
Another aspect of this invention is the product made in accordance with the above described method.
Yet another aspect of this invention, called MIR, i.e., melt irradiation, is a method for making crosslinked UHMWPE. Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded with an inert material that is substantially free of oxygen. The UHMWPE is heated above the melting temperature of the UHMWPE so as to completely melt all crystalline structure. The heated UHMWPE is irradiated, and the irradiated UHMWPE is cooled to about 25xc2x0 C.
In an embodiment of MIR, highly entangled and crosslinked UHMWPE is made. Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded with an inert material that is substantially free of oxygen. The UHMWPE is heated above the melting temperature of the UHMWPE for a time sufficient to enable the formation of entangled polymer chains in the UHMWPE. The heated UHMWPE is irradiated so as to trap the polymer chains in the entangled state, and the irradiated UHMWPE is cooled to about 25xc2x0 C.
The invention also features a method of making a medical prosthesis from radiation treated UHMWPE having substantially no detectable free radicals, the prosthesis resulting in reduced production of particles from the prosthesis during wear of the prosthesis. Radiation treated UHMWPE having no detectable free radicals is provided. A medical prosthesis is formed from this UHMWPE so as to reduce production of particles from the prosthesis during wear of the prosthesis, the UHMWPE forming a load bearing surface of the prosthesis. Formation of the prosthesis can be accomplished by standard procedures known to those skilled in the art, e.g., machining.
Also provided in this invention is a method of treating a body in need of a medical prosthesis. A shaped prosthesis formed of radiation treated UHMWPE having substantially no detectable free radicals is provided. The prosthesis is applied to the body in need of the prosthesis. The prosthesis reduces production of particles from the prosthesis during wear of the prosthesis. In preferred embodiments, the UHMWPE forms a load bearing surface of the prosthesis.
The above and other objects, features and advantages of the present invention will be better understood from the following specification when read in conjunction with the accompanying drawings.